1. Field of the Invention
The present invention relates to a reflective or transmissive image display device for use in a liquid-crystal projector or a projection television set (PTV) for displaying television or computer images at an enlarged scale.
2. Description of the Prior Art
More and more projectors which employ liquid crystal display devices capable of increasing image brightness depending on the brightness of illuminating light sources are finding use in place of projectors which employ cathode-ray tubes (CRTs).
Liquid crystal display devices are roughly classified into reflective and transmissive liquid crystal display devices. In the transmissive liquid crystal display device, illuminating light is applied to a liquid crystal layer on which an image is formed, and the illuminating light which has passed through the liquid crystal layer is projected onto a projection screen by an optical system. In the reflective liquid crystal display device, illuminating light is applied to a liquid crystal layer on which an image is formed, and the illuminating light which has been reflected by the liquid crystal layer is projected onto a projection screen by an optical system.
A color projector using such reflective liquid crystal display devices will be described below with reference to FIG. 11 of the accompanying drawings. As shown in FIG. 11, illuminating white light emitted from a white light source 100 is applied through a beam splitter 101 to a dichroic prism 102, which divides the white light into red light, blue light, and green light that are applied to corresponding reflective liquid crystal display devices 103. Images displayed by respective CRTs 104 are formed on the respective reflective liquid crystal display devices 103, and read as reflections of the applied red light, blue light, and green light. The read light passes through the beam splitter 101, and is projected as a combined image of the three colors onto a projection screen 105.
As shown in FIG. 12 of the accompanying drawings, each of the reflective liquid crystal display devices 103 comprises a pair of glass substrates 111, 118 with transparent electrodes 112, 117 disposed respectively on their confronting surfaces. A photoconductor layer 113 of Si, CdS, or the like, a light shield layer 114, and a mirror layer 115 are successively deposited on the transparent layer 112. A liquid crystal layer 116 is sealed between the transparent layer 117 and the mirror layer 115, thereby assembling a liquid crystal display cell as shown in FIG. 12. A voltage is applied between the transparent electrodes 112, 117.
The image displayed by the CRT 104 is focused onto the photoconductor layer 113 through a focusing lens.
Since the resistance of the photoconductor layer 113 varies depending on the intensity of the light of the displayed image, an electric field applied to the liquid crystal layer 116 also varies depending on the intensity of the light of the displayed image. When illuminating light is applied from the glass substrate 1 18 to the liquid crystal layer 116, an image written in the liquid crystal layer 116 by the focused CRT image is read as reflections of the applied illuminating light.
The color projector shown in FIGS. 11 and 12 requires the three sets of liquid crystal display devices 103 and CRTs 104 corresponding to the three primaries, and is necessarily large in size.
If a color projector has only one liquid crystal display device 103, then the color projector may be reduced in size. One conventional color projector with a single liquid crystal display device uses a mosaic three-primary color filter. However, this conventional color projector utilizes only one-third of the illuminating light. Japanese laid-open patent publication No. 4-60538 discloses a color projector which solves such a problem.
According to Japanese laid-open patent publication No. 4-60538, it is proposed to reduce the size of the color projector without reducing the brightness of illuminating light, using only one liquid crystal display device. In the disclosed color projector, as shown in FIGS. 13 and 14 of the accompanying drawings, illuminating light emitted from a white light source 150 is divided by dichroic mirrors 151 into light rays of three primaries, red (R), blue (B), and green (G), which are applied to a liquid crystal display device 152 at different angles thereto. Light emitted from the liquid crystal display device 152 is projected through a field lens 153 and a projection lens 154 onto a projection screen 155. As shown in FIG. 14, the liquid crystal display device 152 comprises a pair of glass substrates 171, 172 with scanning and signal electrodes 171a, 172a mounted on respective confronting surfaces thereof. A liquid crystal layer 174 is filled in a gap which is defined between the glass substrates 171, 172 by a spacer 173. A planar microlens array 175 is joined to a surface of the glass substrate 171 to which the three-primary light rays are applied. The planar microlens array 175 serves to converge the three-primary light rays onto the signal electrodes 172a (pixel openings).
If the liquid crystal display device shown in FIG. 14 is directly used as a reflective liquid crystal display device, then reflected light does not pass through the centers of the lenses of the planar microlens array 175, as shown in FIG. 15 of the accompanying drawings, so that the illuminating light cannot effectively be utilized.
The lenses and pixels may be arrayed as shown in FIG. 17 of the accompanying drawings for effective utilization of the illuminating light. With the lenses and pixels thus arrayed, the reflected light passes through the centers of the lenses of the planar microlens array 175 as shown in FIG. 18 of the accompanying drawings.
However, as can be seen from FIG. 17, in order for the reflected light to pass through the centers of the lenses, pixel electrodes cannot be arrayed linearly, but must be arrayed in an irregular pattern, which imposes undue limitations on the design of other components, resulting in disadvantages in total design.
The planar microlens array 175 which is employed in the transmissive liquid crystal display device shown in FIGS. 13 and 14 allows almost all illuminating light to pass therethrough. Therefore, it can increase the brightness of images projected onto the projection screen 155. However, because the illuminating light which leaves the liquid crystal display 15 device 152 spreads through a large angle, it is necessary that the projection lens 154 have a large diameter, as shown in FIG. 16 of the accompanying drawings. As a consequence, the entire optical system of the color projector is large in size.
Proposals for reducing the diameter of the projection lens used in combination with the planar microlens array are disclosed in Japanese laid-open patent publications Nos. 5-341283and 7-181487. According to the disclosure of Japanese laid-open patent publication No. 5-341283, as shown in FIG. 19 of the accompanying drawings, a microlens array has two lens arrays 175a, 175b on opposite surfaces of a single glass substrate. The lens array 175a serves to converge illuminating light onto pixel openings, whereas the lens array 175b serves to make principal rays of exiting light parallel to the optical axis thereof. Japanese laid-open patent publication No. 7-181487 reveals two microlens arrays joined respectively to opposite surfaces of a single glass substrate.
If the double-sided microlens array shown in FIG. 19 is incorporated in the optical system shown in FIGS. 13 and 14, then it is necessary that the thickness of the glass substrate 171 be set to such a value as to cause principal arrays of the colors R, B, which are inclined at certain respective angles to the optical axis, to be applied to pixel electrodes 172a corresponding to the colors R, B on the liquid crystal panel. In many cases, pixel pitches are given by liquid crystal panels that are used, and angles at which the light rays of R, G, B are inclined are given by the aperture of the projection lens and the layout of the illuminating optical system, after which the thickness of the glass substrate 171 is determined based on the pixel pitch and the angles. Stated otherwise, the glass substrate 171 may have any of various thicknesses depending on the liquid crystal panel and the illuminating optical system which are used.
The double-sided microlens array shown in FIG. 19 may be fabricated by a process shown in FIG. 20 of the accompanying drawings. According to the illustrated process, a mask 163 is placed over one side of a glass substrate 175 (such as of #7059 or #1737 manufactured by Corning Incorporated or NA45 or NA35 manufactured by NH Technoglass Co. Ltd.), and the glass substrate 175 is etched by isotropic etching to form substantially hemispherical recesses 164 therein. Then, the recesses 164 are filled with a synthetic resin having a high refractive index, producing a microlens array 175a as shown in FIG. 21 of the accompanying drawings. Thereafter, the glass substrate 175 is ground to a desired thickness by a grinding wheel on its surface opposite to the microlens array 175a. The ground surface is then etched by isotropic etching to form substantially hemispherical recesses therein, which are then filled with a synthetic resin having a high refractive index, producing a microlens array 175b (see FIG. 19).
If the ground surface of the glass substrate 175 is not sufficiently smooth but contains a minute flaw, then an etched recess 164 tends to be distorted in shape, as shown in FIG. 22 of the accompanying drawings. The double-sided microlens array with such a distortion has a poor light converging capability. Though the finished glass substrate needs to have any of various thicknesses, as described above, commercially available glass substrates in reality have only certain thicknesses such as of 1.1 mm and 0.7 mm. To process such a commercially available glass substrate into a desired thickness, it is often necessary to grind the glass substrate to a considerable extent, possibly with the need to adjust its thickness according to a rough grinding process, known as lapping, using a loose abrasive material and a hard pad. After the glass substrate has been lapped, it is polished to such an accurate surface finish that any recesses etched in the polished surface will not be distorted. This grinding process is, however, so complex that the manufacturing cost of the double-sided microlens array is high.
The same problem also arises if both surfaces of the glass substrate are initially ground and polished to a desired thickness. As a result the double-sided microlens array shown in FIG. 19 is actually very expensive to manufacture.
The structure disclosed in Japanese laid-open patent publication No. 7-181487, i.e., the microlens array assembly which has two microlens arrays joined respectively to opposite surfaces of a single glass substrate, is also disadvantageous in that its overall thickness is unduly large.